As materials of source/drain electrodes and gate electrodes used in MOS transistors formed on silicon substrates, metal silicides, namely, compounds made of a metal and silicon, are used in order that the resistance of the aforementioned electrodes will be reduced. From the viewpoint of reducing the resistance of gate electrodes and controlling the work functions of materials of gate electrodes, a group of metal silicides, nickel silicides, seems promising as a technology that realizes next-generation semiconductor apparatuses.
When the gate electrode of a P-type MOS transistor is totally made of nickel silicides, ion implantation with P-type impurities in the gate electrode hardly changes the work functions of the nickel silicides. Thus, the flat band voltage cannot be controlled in a P-type MOS transistor that has such a gate electrode.
As a solution to this, the following method has been proposed: when a gate electrode is made of nickel silicides, the work functions of nickel silicides are controlled by composing the gate electrode from nickel silicides that have different kinds of silicide phases without ion implantation with P-type impurities. More specifically, the proposed gate electrode contains Ni3Si2, Ni2Si, Ni31Si12, Ni3Si, and so forth at different composition ratios relative to that of monovalent nickel silicide (NiSi). This is because the work functions of Ni3Si2, Ni2Si, Ni31Si12, Ni3Si, and other polyvalent nickel silicides are closer to that of nickel (Ni) than that of monovalent nickel silicide (NiSi). For this reason, different composition ratios of polyvalent nickel silicides to monovalent nickel silicide (NiSi) allows for the control of the flat band voltage of a P-type MOS transistor.
In an ordinary method for manufacturing a P-type MOS transistor, however, nickel silicides composing the gate electrode is formed via a reaction between polysilicon (P—Si) and nickel (Ni), and thus polyvalent nickel silicides (e.g., Ni3Si) are formed only near the surface of the gate electrode. This is because the diffusion range of nickel (Ni) is probably limited to the vicinity of the surface in ordinary polysilicon (P—Si).
An example solution that can increase the composition ratios of polyvalent nickel silicides (e.g., Ni3Si) in the gate electrode of a P-type MOS transistor is the manufacturing method proposed in Patent Document 1 (Japanese Laid-open Patent Publication No. 2005-294799). According to the description seen in Patent Document 1, the gate electrode of a P-type MOS transistor is formed of polysilicon, portions excluding the gate electrode are masked with a resist, and then polysilicon recedes after reactive-ion etching; after that, nickel (Ni) is deposited on the gate electrode, and then nickel and polysilicon composing the gate electrode are allowed to react with each other by annealing at an appropriate temperature, thereby yielding nickel silicides. In this case, the material layer constituting the gate electrode is thin; thus, the gate electrode is composed of materials rich in polyvalent nickel silicides (e.g., Ni3Si) even though the polyvalent nickel silicides (e.g., Ni3Si) are formed only near the surface of the gate electrode.
In the method for manufacturing a P-type MOS transistor described above, however, it is difficult to change the composition ratios of the polyvalent nickel silicides contained in the portions of the gate electrode materials located near the boundary between the gate electrode and a gate insulating film. This is because such an operation requires precise control of the thickness of the material layer constituting the gate electrode. This means that the work functions of materials constituting the gate electrode are difficult to control. Furthermore, manufacturing processes of CMOS-type semiconductor apparatuses include the step described above only for P-type MOS transistors and thus cannot be simplified.